Abstract

The genus Pinnularia Ehr. (Bacillariophyceae, Bacillariophyta) is very frequently found in algal communities of acidic environments. Several Pinnularia strains isolated from the thermal acidic area of Pisciarelli (Naples, Italy) have been investigated by using a combination of morphological, ecophysiological and molecular techniques and have been compared with 15 Pinnularia strains from acidic and neutral environments, obtained from Loras College Freshwater Algal Collection. Electron microscopy (EM) observations showed the presence of a single Pinnularia species in Italian isolates, Pinnularia obscura. The complete identity between the strains of P. obscura isolated from the thermo acidic hydrosystem of Pisciarelli and from freshwater environments revealed the ability of this species to disperse throughout the globe, colonizing both acidic and neutral habitats. Overall, the genus Pinnularia has been confirmed to show a very weak morphological variation between species; in fact, the main morphological traits helpful to discriminate among them have been identified only by EM. Low pH values were tolerated by the majority of Pinnularia strains and species, both from acidic and neutral environments. As shown by a 18S rDNA phylogenetic analysis, acid-tolerance within Pinnularia is homoplasious, as a reversion from intolerance to tolerance is documented at least in P. gentilis.